Thursday, May 16, 2013

Astatine's Ionization Energy

Astatine is the product of the radioactive decay of some heavier elements and is the rarest naturally occurring element on Earth, only a few grams of astatine is estimated be present in the whole of the Earth's crust at any one time. All of astatine's isotopes are short-lived, with astatine-210 having the longest half-life of all its isotopes, 8.1 hours. As a result, astatine was unknown until 1940, when scientists bombarded bismuth-209 with alpha particles and produced astatine.
Even today we don't know very much about astatine, but estimates about its properties have been made based on its position in the Periodic Table, right under iodine in Group 17 (halogens). The first ionization energy of astatine has been estimated to be between 849.11 and 926.29 kJ/mol (8.8 and 9.6 eV).

The ionization energy, the energy required to remove an electron from the valence shell of an atom, is one of the most important properties that influences the chemical behaviour of an element.

In May 2013 an international team of researchers announced that they had measured the first ionization energy of astatine using laser ionization spectroscopy and found it be be 9.31751 eV (899.02 kJ/mol)

Rothe, S. et al. Measurement of the first ionization potential of astatine by laser ionization spectroscopy. Nat. Commun. 4:1835 doi: 10.1038/ncomms2819 (2013).

Further Reading:

Suggested Study Questions:
  1. Use the Periodic Table to find the following:
    • astatine's chemical symbol
    • astatine's atomic number
  2.  Use the Periodic Table to find the
    • The group astatine belongs to
    • the period astatine belongs to
  3. How many valence electrons does an atom of astatine have? Explain your answer.
  4. Describe the trend in melting points as you go down astatine's group in the periodic table, then estimate the melting point of astatine.
  5. Describe what you think astatine would look like at room temperature and justify your answer on the basis of trends in the periodic table. 
  6. How does astatine-207 differ from astatine-210? 
  7. Astatine-211 and 2 neutrons are produced when bismuth-209 is bombarded with alpha particles.  Write a nuclear equation for this reaction.
  8. Imagine working in a laboratory. You have been given 100 μg of astatine-210 at 9 am for your experiment. Assuming the half-life of astatine is 8 hours, how much astatine-210 would you have left when you leave the lab at 5 pm?
  9. The half-life of astatine-219 is about 1 minute. If you had 100 μg of astatine-219 at 9 am, how much astatine-219 would you have 5 minutes later at 9:05 am ?
  10. Use the information in the article above to estimate the conversion factor between electronvolts (eV) and kJ/mol. 
  11. Why do we not have an accurate measure of how much astatine is found in the Earth's crust?
  12. Suggest a way that scientists can estimate the amount of astatine in existence in the Earth's crust.  

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